Touch

System Specific Notes

Linux: The linux touch system is currently based off event streams, and proc/bus/devices. The active user must be given permissions to read /dev/input/TOUCHDEVICE, where TOUCHDEVICE is the event stream for your device. Currently only Wacom tablets are supported. If you have an unsupported tablet contact me at jim.tla+sdl_touch@gmail.com and I will help you get support for it.

Mac: The Mac and iPhone APIs are pretty. If your touch device supports them then you'll be fine. If it doesn't, then there isn't much we can do.

iPhone: Works out of box.

Windows: Unfortunately there is no windows support as of yet. Support for Windows 7 is planned, but we currently have no way to test. If you have a Windows 7 WM_TOUCH supported device, and are willing to help test please contact me at jim.tla+sdl_touch@gmail.com

Events

SDL_EVENT_FINGER_DOWN: Sent when a finger (or stylus) is placed on a touch device. Fields:

• event.tfinger.touchId - the Id of the touch device.
• event.tfinger.fingerId - the Id of the finger which just went down.
• event.tfinger.x - the x coordinate of the touch (0..1)
• event.tfinger.y - the y coordinate of the touch (0..1)
• event.tfinger.pressure - the pressure of the touch (0..1)

SDL_EVENT_FINGER_MOTION: Sent when a finger (or stylus) is moved on the touch device. Fields: Same as SDL_EVENT_FINGER_DOWN but with additional:

• event.tfinger.dx - change in x coordinate during this motion event.
• event.tfinger.dy - change in y coordinate during this motion event.

SDL_EVENT_FINGER_UP: Sent when a finger (or stylus) is lifted from the touch device. Fields: Same as SDL_EVENT_FINGER_DOWN.

Functions

SDL provides the ability to access the underlying SDL_Finger structures. These structures should never be modified.

The following functions are included from SDL_touch.h

Devices are tracked by instance ID, of type SDL_TouchID.

To get a list of available device SDL_TouchID values, call SDL_GetTouchDevices(). This returns an array of device IDs, terminated by a zero ID. Optionally, you can get a count of IDs by passing a non-NULL int* to SDL_GetTouchDevices() if you'd rather not iterate the whole array to get this number.

A SDL_TouchID may be used to get pointers to SDL_Finger.

SDL_GetNumTouchFingers(touchID) may be used to get the number of fingers currently down on the device.

The most common reason to access SDL_Finger is to query the fingers outside the event. In most cases accessing the fingers is using the event. This would be accomplished by code like the following:

  float x = event.tfinger.x;
  float y = event.tfinger.y;

To get a SDL_Finger, call SDL_GetTouchFinger(SDL_TouchID touchID, int index), where touchID is a SDL_TouchID, and index is the requested finger. This returns a SDL_Finger *, or NULL if the finger does not exist, or has been removed. A SDL_Finger is guaranteed to be persistent for the duration of a touch, but it will be de-allocated as soon as the finger is removed. This occurs when the SDL_EVENT_FINGER_UP event is added to the event queue, and thus before the SDL_EVENT_FINGER_UP event is polled. As a result, be very careful to check for NULL return values.

A SDL_Finger has the following fields:

• x, y: The current coordinates of the touch.
• pressure: The pressure of the touch.

Please direct questions/comments to: jim.tla+sdl_touch@gmail.com (original author, API was changed since)